Cerebellar, medullary and spinal afferent connections of the paramedian reticular nucleus in the cat

BrainResearch, 332(1985)267-282 Elsevier

267

BRE 10678

Cerebellar, Medullary and Spinal Afferent Connections of the Paramedian Reticular Nucleus in the Cat K V ELISEVICH, A W HRYCYSHYN and B A FLUMERFELT Department of Anatomy, Health Sciences Centre, The UntversIO' of Western Ontarzo, London N6A 5C1 (Canada) (Accepted July 23rd, 1984) Key words paramedlan reticular nucleus - - deep cerebellar nuclei - - vestibular nuclei - - cervical cord - nucleus lntercalatus - - sohtary nucleus - - horseradish peroxldase

The topographic organization of afferent projections from the deep cerebellar nuclei, medulla oblongata and spinal cord to the paramedlan reticular nucleus (PRN) of the cat was studmd using the horseradish peroxldase (HRP) method of retrograde labelling Discrete placements of HRP within each of the dorsal (dPRN) and ventral (vPRN) regions of the PRN showed some segregation of input The deep cerebellar nuclei project in a predominantly contralateral fashion upon the PRN A small but significant lpstlateral fastlgial afferent component is also present The fastlgial and dentate nuclei contribute the majority of fibers to the dPRN whereas the interposed nucleus provides very little The vPRN receives a relatively uniform input from all 3 cerebellar nuclei Both lateral vestibular nuclm contribute the majority of fibers from the vestibular nuclear complex largely from their dorsal division Additional input arises from bilateral medial and inferior vesnbular nuclei The vPRN recewes relatively more fibers from the inferior vestibular nuclei than does the dPRN while inputs from the medial vestibular nuclei are comparably sparse The PRN receives bdateral projections from the nucleus lntercalatus (of Staderml) A significant projection to the contralateral PRN occurs from the ventrolateral subnucleus of the sohtary complex and Its immediate vicinity Additional sources of medullary afferent input include the lateral, glgantocellular and magnocellular tegmental fields, the contralateral PRN and the raphe nuclei Sites of ongm of spinal afferents to the dPRN are bilaterally distributed mainly within Rexed's laminae VII and VIII of the cervical cord whereas those to the vPRN are confined largely to the medial portion of the contralateral lamina VI in the C1 segment A few labelled cells are found m the thoracolumbar cord with those to the vPRN being more caudal These data provide the neuroanatomlcal substrate for a better understanding of the functional role of the PRN m mediating cardiovascular responses approprmte to postural changes

INTRODUCTION

shown e l e c t r o p h y s l o l o g l c a l l y to r e c i p r o c a l l y c o n n e c t with the P R N 47 and its n e u r o n s a p p e a r also to re-

T h e p a r a m e d i a n r e t i c u l a r n u c l e u s ( P R N ) of the

spond to slnusoidal s t i m u l a t i o n of n e c k and labyrinth

caudal m e d u l l a o b l o n g a t a is a p r e c e r e b e l l a r relay nu-

receptorsS

cleus that r e c e i v e s i m p o r t a n t inputs f r o m the d e e p c e r e b e l l a r nucleP, 15,17,2<35~56 and v e s t i b u l a r nu-

t h r o u g h the F N (fastigial p r e s s o r r e s p o n s e ) and the

clelm 37 A f f e r e n t s f r o m the cervmal c o r d h a v e also b e e n p o s t u l a t e d b a s e d u p o n d e g e n e r a t i o n studies fol-

Cardiovascular

responses

medmted

carotid sinus n e r v e ( b a r o r e c e p t o r reflex) h a v e also b e e n i n f l u e n c e d by e x c i t a t i o n or ablation PRNS,6,14,23,29-31,41-46

of the

lowing lesions of the dorsal c o l u m n s in the cat 10.

V a r i o u s aspects of t h e p a t h w a y s f r o m the d e e p cer-

Physiological i n v e s t i g a t i o n s h a v e i n d i c a t e d that the

ebellar nuclei to the P R N h a v e b e e n s t u d m d using lesion techniquesZ5,54 56 and the a u t o r a d l o g r a p h m tra-

P R N r e s p o n d s to the d e g r e e and d i r e c t i o n of h e a d and t r u n k tilting xn a specified p a t t e r n e d fashion similar to that o b t a i n e d within the v e s t i b u l a r n u c l e a r

cing m e t h o d 3. U s i n g tritiated a m i n o acids, B a t t o n et al. 3 f o u n d that rostral parts of the F N , in p a m c u l a r ,

c o m p l e x and the rostral fastigtal nucleus ( F N ) of the c e r e b e l l u m 2053 T h e i n t e r p o s e d n u c l e u s has b e e n

a p p e a r e d to c o n t r i b u t e all of the fastigial fibers to the c o n t r a l a t e r a l P R N via the u n c i n a t e fasclculus Stml-

Correspondence K Ehsevlch, Department of Anatomy, Health Sciences Centre, The University of Western Ontario, London, Ont , Canada N6A 5C1 0006-8993/85/$03 30 © 1985 Elsevier Science Pubhshers B V (Biomedical Division)

2{~8 larly, injections oI trltmted /euclne confined to the dentate and interposed nuclm resulted m some terror nal labelling in the region ol the PRN 2~ Lesmns ol the brachlum conjunctlvum have produced a few, degenerating terminals in the contralateral PRN2~ Degenerated fibers have been found m the PRN bilaterally after extensive lesions of the vestlbulai complex m,33,37 although fastlgloretlcular fibers may have been Interrupted by the lesions. Pretermlnal fiber degeneration in the PRN after destruction of the nucleus of the solitary tract (NTS) ze supported pre~ lOUS assumptmns of a relay through the NTS from the carotid sinus nerve to the P R N after electrical stlmulatmn of the latter nerve 5 6,> None of the abo~e studies have been confirmed or supported by the H R P retrograde tracing method The present study was undertaken to investigate the details of projection from the deep cerebellar nuclei and the vestibular nuclear complex upon the P R N and to establish the origin of other medullary and spinal afferent input to the PRN using discrete placements of H R P within the PRN of the cat. MATERIALS AND METHODS A total of 20 cats, of either sex, weighing 2 0 - 4 7 kg, were used in this study Anesthesia was induced bv a mixture of ketamine hydrochlorlde (Ketaset, Rogar/STB, London, Ont.) (10 mg/kg, i m.) and xylazlne (Rompun, Cutter Laboratories. Mlsslssauga, Ont.) (2,2 mg/kg, 1 m.) and maintained by sodium pentobarbltal (Somnotol, M.T C. Pharmaceuticals, Hamilton, Ont.) (6.5 mg/30-60 rain l v.) A pre- and postoperative dose of benzathlne penlcllhn G (50,000 units, i m.), procaine pemcilhn G (50,000 units, 1.m.) and dihydrostreptomycin (125 mg, 1 m ) (Penlong S, Rogar/STB, London, Ont.) was given Fine glass pipettes with a tip diameter of 30-50/~m o.d were prepared from Klmax glass tubing. The pipettes were filled by capillarity with a concentrated solution of H R P conjugated with wheat germ agglutlnln ( W G A - H R P ) (Sigma, Type VI, St. Louis). In 5 of the 20 animals, a solution combining both W G A H R P and unconjugated H R P (Sigma, Type VI. St Louis) in a 50:50 mixture was used The solution was allowed to dry at the tip by exposure to air for 1-2 h, thus forming a sohd pellet of crystallized H R P The pipette was fixed to a dissecting needle and mounted

m a DKI stereotaxlc apparatus I he lh~,r ,)t ~he ~.ul dal portum of the fourth \entrlcle ~ . . cxposc~ by forward displacement of the cerebelhml ,diet occipital cranlectomy The pipette was introduced ,t', a 41r angle relative to the floor o! tile ~entncJc . m m lateral to the mMhne to avoid damaging tibe~ oI descendlng spmal projections and p o s m o n e d . , the PRN transependymally using the obex as an external landmark It was left In place Ior 2 - 5 mm t,~ allow the crystallized H R P to dissolve m the tissue fhnd The size ot the deposit was readily controlled by an apploprlate combination of pipette diamete~ and placement time 2J Following postoperahve surx~al tHnes of 3 - 5 days, the ammals were reanesthetlzed Wlttl ketamine hydrochlorlde (20 mg/kg, l m I and sodmm pentobarbltal (40 mg/kg, l \ ) and perfused transcat&ally using. (1) 500-800 ml ot 0 9% NaCI at 20 '-C for 8 mln, (2) 2000 ml of 0 1 M phosphate buffer (pH 7 4) containing 1% paraformaldehyde (Fisher) and I 25% glutaraldehyde (Fisher) at 20 °C tor 30 rain, and (3) 2000 ml of 0.1 M phosphate buffer (pH 7 4) containing 10¢; sucrose at 4 °C tor 31) mm Fhe brain and spinal cord were removed and stored m pertuslon solution (3) at 4 °C for 12-36 h Subsequently, frozen serial sections through the cerebellum, medulla and spinal cord segmental areas C 1 - 3 , {'5, C8. T2. T4, T7, T10, L1, L3, L7 and $2 were cut with a sledge mlcrotome at thicknesses of 40 f~m The cerebellum was sectioned saglttally and the bralnstem transversely One from every 2 serial sections through the mjectmn site and 1 from ever9 5 serial sections through the remaining tissue was placed in 0 ] NI phosphate buffer (pH 7 4) at 4 °C, rinsed 3 times with dmomzed water and processed according to the method of Mesulam ~ and Mesulam et al a,, using tetramethyl benzldine (TMB, Sigma, St Lores) as the chromogen Sections were then mounted on chrome-ahim gelatinlzed shdes and counterstalned with 0 ]~/c Neutlat red solution (pH 4.8) for 3 mln and wewed under bright- and dark-field illumination Both the lnlcction site and the distribution of labelled cells were mapped from representative sections for each animal Frozen transverse sections of cervical, thoracic and lumbar segments of the spinal cord were examined for anterograde unconjugated H R P label within the terminals of expected hbers of passage through the site of H R P injection m the 5 annnals rejected with a mixture of W G A - H R P and unconlugatcd

269 HRP Intact fibers of passage have been shown to transport unconjugated HRP anterogradely 13 Labelled terminals of me&al vestibulospinal fibers were looked for In Rexed's laminae VII and VIII in the cervical and upper thoracic segments. Additionally, labelled terminals of tectosplnal fibers in the upper cervical segments (laminae VI and VII) and lnterstltiosplnal fibers throughout the cord (laminae VII and VIII) were looked for Absence of anterograde label in the expected sites of termination of these fibers of passage supported the premise of primary uptake by terminals in the region of the PRN RESULTS

Description of rejection sltes To study the topographic distribution of afferent input to the PRN, the dorsal (dPRN) and ventral (vPRN) subdivisions of the nu+leus were separately injected with W G A - H R P or a mixture of WGAHRP and free HRP Retrogradely labelled neurons were identified by a granular blue reaction product within their perikarya following conversion with TMB The dPRN was inlected with only W G A - H R P in 8 cats and with a mixture of W G A - H R P and free HRP in another 3 cats The vPRN was injected with only W G A - H R P in 7 cats while the remaining 2 cats received the mixture The following description is typical of most of the lnlectlon sites within either the dPRN or vPRN The injection site consisted of a centrally placed pipette tract surrounded by a central core of dense deposit of HRP In each case, the effective zone of uptake in the dense core was confined to either division of the PRN. A zone of secondary diffusion which produced a lightly stained halo around the dense central deposit was found in some cases to extend medially towards the medial longitudinal fasciculus, rostrally and dorsally towards the perlhypoglossal nuclei or ventrally to the inferior olive These sites of secondary diffusion have been shown not to contribute significantly to retrograde neuronal labelhng32 55 Furthermore, LaVail and LaVal134 have shown that HRP uptake by damaged fibers of passage within the diffuse periphery of injection sites does not occur In the present study, retrograde neuronal labelling was therefore considered to result from uptake within the dense core of the HRP injection site

The pattern of retrogradely labelled cells identified after injection of either W G A - H R P or the HRP mixture did not differ appreciably for either the dPRN or vPRN. Although anterograde label was identified in some cases, the pattern was light indicating minimal uptake by fibers of passage

(t) Deep cerebellar nuclet Neurons within all three contralateral deep cerebellar nuclei and the lpsllateral fastxglal nucleus (FN) were labelled following separate injections of WGAHRP into the dPRN and vPRN No eerebellar cortical labelling was noted (a) Fasugtal nucleus. In the case of dPRN Injections, most labelled neurons in the contralateral FN appeared within its dorsomedlal portion (Fig. la) A large number of these cells were concentrated posteriorly with relatively few cells found scattered throughout the entire anterior aspect of the FN Labelling in the ipsllateral FN was relatively sparse and concentrated within the ventromedlal aspect of the anterior half of the nucleus The labelled perlkarya were mostly small or medium-sized, round and mulnpolar neurons with a few scattered larger labelled cells found throughout (Fig 2a) Injection of the vPRN resulted in relatively more cell labelling along the anterior aspect of the medial half of the FN in addition to labelling of cells in Its dorsal half (Fig 3a) Labelled neurons in the lpsllateral FN were again less abundant and largely confined to its ventromedlal portion (b) Interposed nucleus. A paucity of labelled cells within the interposed nucleus (IN) was noted after dPRN lnlectlons and these were confined to its posterior subnucleus (INp) (Fig lb) InJection of the vPRN resulted in more abundant neuronal labelling in both the anterior (INa) and posterior (INp) subnuclel (Fig 3b) Most of the labelled cells were concentrated in the anterodorsal portion of the INp with fewer scattered labelled cells found posteriorly in the INa. Most labelled cells were medium-sized and multlpolar (Fig 2b) (c) Dentate nucleus Labelled cells were found throughout the anteroposterlor extent of the dentate nucleus, largely central with respect to its medial and lateral borders (Figs. lc, 2c and 3c) This was the case with both dPRN and vPRN injections although in the latter case, relatlvelv fewer labelled cells were

270

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Fig 1 Distribution of labelled neurons in the contralateral deep cerebellar nuclm and lpsllateral fasnglal nucleus (inset) after rejection of the right dPRN (photommrograph). Numbers mdmate approximate distances of saglttal levels from the rmdhne m mm Each dot represents a single cell and each level mdmates the cumulative labelled cell population of four tlssuc secUons (a) FN, (b) DN and INp, (c) DN

271 encountered. Many cells were lightly labelled in comparison to the previous nuclei.

(u) Vesubular nuclei The majority of labelled vestibular neurons were found in both lateral vestibular nuclei after separate injections of either the dPRN or the vPRN (Figs 4 and 5; Table I). A larger number of neurons was labelled within the dorsal division (VLd) than within the ventral division (VLv) of both nuclei and a heavier concentration of labelled cells was found after injection of the dPRN than of the vPRN The giant cells of the VLd and the large and medium-sized multtpolar cells of the VLv were labelled Relatively fewer neurons within the inferior and medial vestibular nuclei of both sides were hghtly labelled (Figs. 4, 5, 6c, d, e). Of the latter two nuclei, labelling was more frequent in the lpsllateral inferior vestibular nucleus particularly after inlectlons of the vPRN (Table I). No labelled cells were found in the superior vestibular nuclei

(ut) Non-vestibular medullary nuclei (a) Solitary nuclear complex Injection of the dPRN resulted in labelling of cells mainly within the opposite ventrolateral sohtary nucleus (Svl) and a

few scattered cells of the commissural and intermediate solitary nuclei (Fig. 7a). Injection of the vPRN resulted in less marked labelling of cells in both the contralateral and ipstlateral Svl although more labelled cells appeared in the area immediately ventral to the solitary complex (Fig 7b) The proJection to the PRN from the solitary complex was heavier from its caudal half. Labelled neurons were mainly small and medium-sized and appeared spindle-shaped or multipolar (Fig 6a) (b) Nucleus intercalatus (of Stadermt) Labelled neurons appeared in approximately equal numbers in the nucleus lntercalatus of both sides when either the dPRN or vPRN was injected (Figs 4 and 5). These appeared generally throughout most of the rostrocaudal extent of the nucleus. Injection of the dPRN produced a distribution of~ labelled cells within the nucleus lntercalatus which extended further rostrally than did that of the vPRN The labelled cells were usually small while only a few were medium-sized (Fig 6b), Most had oval cellular outlines or were fusaform in shape. (c) Raphe. Labelling of large, medium-sized and small fuslform cells occurred throughout most of the inferior central nucleus (nucleus raphe obscurus) with injection of both the dPRN and vPRN, the latter

) Fig 2 Photomicrographs(bright-field microscopy)of labelled neurons in the deep cerebellar nuclm ( x 470) (a) left FN, (b) INp, (c) DN

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Fig 3 Distribution of labelled neurons m the contralateral deep cerebellar nuclei and lpsdateral fastlglal nucleus (reset) after rejection of the right vPRN (photomicrograph) Numbers indicate approximate distances of sagtttal levels from the mldhne m mm Each dot represents a single cell and each level indicates the cumulative labelled cell population of four tissue secllons (a) FN. (b) INa and INp, (c) DN

273

ROSTRAL

CAUDAL Fig 4 D t s t n b u t l o n of labelled neurons m the medulla oblongata after reJection of the d P R N Each dot represents a single cell and each level indicates the cumulative labelled cell population of four tissue sections

274

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CAUDAL qg 5 Distribution of labelled n e u r o n s m the medulla oblongata after m l e c n o n of the v P R N Each dot represents d ~mgle cell 'ach level indicates the c u m u l a n v e labelled cell p o p u l a n o n of four nssue sections

275 TABLE I

Number of labelled neurons within medullary nuclet after mlectton o¢ the d P RN (C-22) and v P R N (C-31) Nucleus

dPRN (C-22)

vPRN (C-31)

Left

Right

Total

Left

Rtght

Total

INT VLd VLv VMN VIN FTM FTG

13 13 I/ 5 2 4 13

12 24 4 9 6 6 13

25 37/4l 4J 14 8 10 26

10 6 1 5 7 11 14

9 9 3 6 20 23 21

19 15/ 19 4/ 11 27 34 35

Sm Corn

7 8

/I 5

0 2

0 0

12

40 13)

resulting m a larger number of labelled cells m the raphe In addition, the postpyram~dal nucleus of the

raphe (nucleus raphe palhdus) contained labelled cells resulting from injection of the vPRN (Fig. 5). Labelling of cells appeared throughout the rostrocaudal extent of the medullary raphe but was concentrated more m its caudal portion, a reflection perhaps of inJury to traversing fibers near the sxte of injection (d) Remammg nuclet Labelled neurons appeared within the contralateral dPRN and vPRN after injection of either the dPRN or vPRN Medmm-slzed multlpolar and polygonal cells were most often labelled. Sparse neuronal labelhng of the mfracerebellar nucleus on both s~des appeared after mlectton of the dPRN Lateral (FTL), glgantocellular (FTG) and magnocellular (FTM) tegmental fields ~psllateral and contralateral to the site of mlect~on also contained labelled neurons (Table i) These were mainly large or medmm-sized and multlpolar m appearance An f

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Fig 6 Photomicrographs (bright-field microscopy) of labelled neurons m the medulla oblongata and cervical cord ( x 470) (a) left NTS, (b) right INT (c) right VMN, (d) right VIN, (e) left VLd, (f) right lamina VI, (g) right lamina VII

27(~ CAUDAL

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vlous mject~ons of the P R N could haxc > , u l t e d f r o m m t e r r u p n o n of fibers t r a v e r s m g the a l c a ~,~ t n l e c t u m .

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side after m l e c t l o n of the v P R N ltk¢]~ ~c~ulted f r o m i n t e r r u p t i o n of fibers ot the m e d t a l l e m m s c u s and ohv o p e t a l fibers,

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Fig 7 Distribution of labelled neurons m the nucleus of the sohtary tract and lmmedmte area following mjecnon of (a) the right dPRN, and (b) the right vPRN Each dot represents a single cell and each level indicates the cumulative labelled cell population of four tissue sectmns H R P i n j e c t i o n of the lateral t e g m e n t a l field at the level of the P R N r e s u l t e d m a n t e r o g r a d e as well as r e t r o g r a d e labelling of s o m e cells w i t h m the lpsilateral and c o n t r a l a t e r a l P R N , c o n t r a l a t e r a l F T L and the r a p h e m d l c a t m g that at least s o m e of the labelled cells seen within the P R N , F T L and r a p h e on pre-

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Fig 8 Dlstnbunon of labelled neurons m the spinal cord fop lowing reJection of (a) the right dPRN, and (b) the right vPRN Each dot represents a single cell and each segmental level indicates the cumulative labelled cell population of ten t~s',He ~ectlorls

277

(tv) Spinal cord The majority of retrogradely labelled spinal neurons were found m the cervical cord with inlectmn of either the dPRN or vPRN In the case of the dPRN, labelled cells were distributed bilaterally in approximately equal proportions in laminae VII and VIII (Fig. 8a). A few addmonal scattered labelled cells were found in laminae V and VI of the cervical cord as well as laminae VI, VII and VIII of the upper thoracic cord Injection of the vPRN produced labelling of cells which was concentrated largely in the medial zone of lamina VI in the contralateral C1 region of the cervical cord (Fig. 8b) A few additional labelled neurons appeared bilaterally in laminae VII and VIII of the remaining cervical and upper thoracic cord as well as the lower thoracic and upper lumbar cord. Small labelled cells appeared largely within the medial zone of lamina VI and were oval or triangular in shape (Fig. 6f) Most labelled' cells within laminae VII and VIII were medium-sized and triangular or spindle-shaped (Fig. 6g). DISCUSSION Afferent projections to the dorsal and ventral subdlvissons of the PRN differ somewhat m the origin and magnitude of their Inputs. On the whole, however, they largely coincide and can be summarized as follows. Deep cerebellar nuclei A cerebelloretlcular contribution to the PRN was first described using the silver impregnation method of Glees and shown to be bilateral and moderate in amount m Since then, restricted lesions of the deep cerebellar nuclei and improved tracing techniques have refined their efferent distribution further Fasttgtal nucleus. Bilateral fastiglobulbar prolections to the PRN in the cat were shown using the Marchi and Nauta methods 15,54. Terminal degeneration in the medullary reticular formation was chiefly medial and contralateral exceeding that found in both the lateral reticular nucleus and perlhypoglossal nuclei. Degeneration within the dPRN exceeded that of the vPRN after complete destruction of the contralateral FNS0 Slightly less intense degeneration was found Ipsllaterally and this may have resulted from damage of the neighboring unclnate fasciculus carrying fibers of the contralateral FN. Autoradlographic

studies performed in the monkey later showed fastlgloretlcular fibers to be almost entirely crossed, passing wa the unclnate fasclculus and arising from all rostrocaudal levels of the FN except for its extreme rostral pole 3. Labelled fibers within the lpsilateral juxtarestiform body were not found to terminate in the PRN. Rostral parts of the FN and, in particular, dorsolateral regions of the rostral half of the FN, appeared to contribute all of the fibers destined to the PRN Earlier work in the opossum using the FlnkHelmer method following lesions confined to the posteromedial FN revealed terminal degeneration in the PRN 35. This contrasted with the results obtained after lesions of the anterolateral FN in which no terminal degeneration was found in the reticular formation Electrical stimulation of either FN produced excitatory postsynaptlc potentials with monosynaptlc latencles in cells within the area of the PRN al Furthermore, the presence of the PRN appeared necessary in mediating the fastlgial pressor response upon stimulation of the ventromedlal portion of the rostral half of the FN in the cat 44 Bilateral lesions of the PRN completely abolished the response whereas unilateral ablation reduced the amplitude of the response by one-halP 2 a4 The results of the present study are In partial agreement with previous anatomical work in that a bilateral afferent contribution to the PRN from the FN is evident in the cat The fastiglal contribution to the dPRN is predominantly contralateral and found dorsomedlally with a posterior concentration in the FN similar to the opossum 35 A similar pattern is found in the case of the vPRN although relatively more fastigial fibers originate anteriorly within the nucleus The ventromedlal portion of the anterior half of the FN contributes the majority of fibers to the lpsIlateral PRN and may coincide with the location of cells responsible for the fastlglal pressor response mediated through the PRN J'4,45, The findings differ somewhat with those found In the monkey ~ and this may perhaps be resolved by species differences and the lower sensitivity of the autoradlographic technique In the case of the sparser lpsllateral FN proJection Fastlgiosplnal fibers pass ventrolaterally within the medulla 3 and could not have been interrupted by the lnlectlon procedure

278

Interposed and dentate nu~ lel

noted using the F m k - H e i m e r technique after destruction of the contralateral b r a c h m m conjuncnvuin :~ A significant afferent projection to the P R N from the D N is confirmed in this study No difference in the topographical pattern of afferent input lrom the DN to the d P R N and v P R N ~s evident Th~s disagrees with earlier findings of terminal degeneration found mainly about cells of the vPRN after lesions of the DN in the cat~: The P R N is also shown to be one of the most caudal sites of termination of afferent fibers from the IN In particular, a m a r k e d afferent input to the v P R N is noted relative to that destined for the d P R N The sparse crossed spinal prolectlon of the INpX would not have been i n t e r r u p t e d by injectmn of the PRN. The findings are m a g r e e m e n t with previous electrophyslological work describing a prolectlon from the IN to the P R N m which neurons m the IN were activated antldromlcally from the contralateral P R N 47 The present study indicates that most of these neurons are hkely to be found m the l N p and to prolect to the vPRN

difficult)', confirms the existence ot a r,llateral ~esllbuloretlcular projection with an lpsdatc~rai predomNnance The ventrolateral p o s m o n ol tile lateral ~cstTbulospinal fibers within the medulla a~ 01ds tile problem of inadvertent Injury o[ fibers of passage during rejection of the P R N The same cannot bc said for the descending fibers of the V M N and VIN x~hlch course medmlly within the medulla although their prolcctlons to the P R N a p p e a r to be m excess, oI their relatively sparse spinal projections In addition, examination of cervicothoraclc spinal cioss-seCtlOnS revealed no anterograde label m the site, of termination of medial vestibulospmal fibers (1 e laminae V l l and VIII), supporting the premise that ~er). lew if any medial vestibulospinal fibers had been dam~ aged in the penetration There Is no evidence for an afferent input from the superior vestibular nucleus to the P R N A topographical pattern appears to be present and ~s of interest in tile functional orgamzation of the P R N (Table I) In particular, mediation ot extensor tone of the limbs b~ the V k N ~H ma~ have some bearing upon the involvement ol the P R N m postural control Neurons of the PRN have been shown to respond to lateral tilt of the head and trunk m a pattern s~mflar to that seen amongst neurons of the vestibular nuclei and the FN 2~' ~ h k e w l s e , the V M N mav have similar influences as n has been shown to project via the medml vestlbulospmal tract onto the m o t o r neurons ol the upper cer~,lcal cord 5~' The demonstration of afferent input horn the VIN establishes a descending supraspinal prolectlon from this nucleus ProJections of the VIN to both fastiglal nuclel~ may have some bearing on afferent input from the latter to the P R N

Vestibular nuclei

Remammg medullary nuclez

Lesions of various portions of the vestibular nuclear complex m the cat p r o d u c e d terminal degeneration mainly within the Ipsdateral v P R N and less m a r k e d l y in the dPRNW.l~.3~ In the m o n k e y , lesmns of the medial vestibular nucleus p r o d u c e d modest pretermlnal degeneration m the P R N with an lpsflateral p r e d o m i n a n c e 37 Definite conclusions regarding the vestIbuloretlcular projection have been hamp e r e d by concomitant destruction of fastigloreticular fibers which are known to course through the lateral (VLN), medial (VMN) and inferior (VIN) vestibular nuclei 3 The present study, while avoiding the latter

The demonstration of a distinct afferent projection from the caudal solitary nucleus (NTS) supports earher findings of p r e t e r m m a l fiber degeneration m the P R N after lesions of the NTS m the cat:: Previous work has failed to d e m o n s t r a t e a similar p r o l e c n o n using the F m k - H e l m e r method49 and autoradlography in the r a t ~ It is n o t e w o r t h y that H u m p h r e y 3~ recorded long latency evoked potentials from the region of the P R N during stimulation ol the carotid sinus nerve (CSN), a buffer nerve which projects mainly to the NTS, and a t m b u t e d the delay of afferent input to synaptlc transmission through the NTS

A c o m m o n m a m m a l i a n pattern has been shown m the overall d l s m b u t l o n of cerebellofugal fibers iormlng the contralateral descending p a t h w a y of the brachium conjunctlvuin as e~ldenced m the rat j~. opossum,S 365~ cat1127 and m o n k e y ~s Evidence of projections from the dentate (DN) and interposed (IN) nuclei to the P R N m the cat has been conflicting ~v:~ InJection of t n t i a t e d leucme confined to the DN and IN p r o d u c e d some labelling m the PRN :7, however, application of H R P into the P R N m the opossum failed to r e t r o g r a d e l y label cells in the DN and IN )~' Terminal degeneration in the P R N of the rat was

279 More recently, recordings by lntracellular methods of mono- and pauc~synaptic potentials from neurons within the P R N have been m a d e upon electrical stimulation of the CSN showing synaptm activation to occur largely in the dorsolateral q u a d r a n t of the P R N al, Some topographical differences in the connections of the NTS with each of the d P R N and v P R N are apparent in this study W h e r e a s the contralateral ventrolateral solitary nucleus (Svl) projects mostly onto the d P R N , the v P R N receives a lesser input from both the contralateral and ~psilateral Svl In additmn, however, it receives an input from an area immediately ventral to the NTS This latter site of efferent projection to the P R N corresponds to another terminal field of p r o j e c t m n of the CSN 19 implying a second source of relayed b a r o r e c e p t o r and/or c h e m o r e c e p tor information to the PRN. Collaterals of cerebellar afferents from the NTS may project to the P R N Of interest is the finding of similar terminal fields in the anterior lobe vermis of the efferent projections of both the P R N 1~ 51 and the caudal NTS 52. This may implicate the P R N in a m o d u l a t o r y circuit involving the NTS and cerebellum A significant bilateral afferent m p u t from the nucleus lntercalatus of Staderlm (INT) to the P R N has not been previously r e p o r t e d It shows no particular topographical a r r a n g e m e n t and originates throughout most of the rostrocaudal extent of the nucleus The INT itself receives projections from the V M N and VIN ~s some of which may themselves be collaterals of fibers from the latter two nuclei to the P R N . The medial part of the NTS also Is known to project to the INT 49 and a similar a r r a n g e m e n t may exist These lnterconnectlons illustrate the complexity of the circuitry involving primary or collateral projections to the P R N at the level of the caudal m e d u l l a suggesting by itself a possible m o d u l a t o r y role for the P R N at the p r e c e r e b e l l a r and presplnal level The presence of an afferent input from the lateral tegmental field (FLT) is of some interest as it hes within the cardiovascular pressor area of the m e d u l l a 1 and a monosynaptlc input from the CSN to the F T L has more recently been d e m o n s t r a t e d 41 The magnitude of the projection from the F T L cannot be defmltely stated because of the certain interruption of some of its efferent fibers traversing the P R N to other destinations. Similarly, some efferent projections of the raphe are likely to have been Interrupted as they tra-

versed the P R N The present study in part confirms previous autoradiographic evidence of an afferent contribution from the p o s t p y r a m l d a l nucleus of the raphe (nucleus raphe palhdus) to the v P R N 7. The efferent projections of the inferior central nucleus of the raphe have been httle studied and their terminations are not yet accurately stated The results of the present study suggest that the P R N is possibly one of these either as the recipient of a primary input or a collateral input of the p r o m i n e n t ascending projection of this nucleus A bilateral input from the gigantocellular ( F T G ) and magnocellular (FTM) tegmental fields has not been previously r e p o r t e d The projection is somewhat heavier lpsilaterally particularly that to the v P R N Of interest is an efferent projection from the NTS to the F T G 49 along which the PRN may be a collateral station

Spmal cord A m o d e r a t e spinal projection to the P R N traversing the dorsal funlcuh has previously been reported m. Convincing evidence of a relay in the dorsal column nuclei of spinal impulses influencing the PRN has not been forthcoming2, 4 10 28 Neither can it be said to occur m the present study Stimulation of cutaneous m e c h a n o r e c e p t o r s of the forelimb and handlimb in the cat has excited neurons in the PRN 24 The present work identifies neurons primarily in the cervical cord projecting to the P R N and establishes in part the anatomical substrate for the previous findlngs. Projections to the v P R N begin further caudall.~ within the t h o r a c o l u m b a r cord whereas those to the d P R N are almost entirely confined to the cervical cord. These connections imply differences in the receptive fields of the two regions in the P R N and may have some bearing on their respective roles in modulating postural information at the p r e c e r e b e l l a r stage In summary, afferent inputs to the dorsal and ventral divisions of the P R N arise from a n u m b e r of sites within the medulla, cerebellum and spinal cord. A topographical organization is a p p a r e n t with some separation of input to either division of the PRN based largely on relative magnitudes These data provide supportive anatomical evidence for the P R N ' s role in cardiovascular and postural control

280 ACKNOWLEDGEMENTS

A. B e n t l e y / o r t y p i n g t h e m a n u s c r i p t

!-Ins w o r k ~ , ~

s u p p o r t e d bv t h e M e & c a l R e s e a r c h ~. ,),mcfl t)l ( a T h e a u t h o r s wish to t h a n k M r s

J

Sholdlce and

nada.

M r s S G r i f f i n for t h e i r t e c h n i c a l a s s i s t a n c e a n d Mrs

ABBREVIATIONS 5SP 5SM 6M 7M 12M AMB AP C CC C1 Com CSN CX dPRN DMN DN FN FI'G FTL FI'M G HRP IFC IN

spinal mgemmal nucleus, parvocellular dmvismn spinal mgemmal nucleus, magnocellular &vlsmn abducens nucleus facml nucleus hypoglossal nucleus nucleus amblguus area postrema cuneate nucleus central canal interior central nucleus of the raphe commissural sohtary nucleus carotid sinus nerve external cuneate nucleus paramedlan reticular nucleus, dorsal dwlsmn dorsal motor nucleus of the vagus dentate nucleus fastlgml nucleus glgantocellular tegmental field lateral tegmental field magnocellular tegmental field gracde nucleus horsera&sh perox~dase lnfracerebellar nucleus interposed nucleus

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